1. Field of the Invention
The present invention relates to a monolithically integrated alignment device for coupling optical energy between optical devices and a method for producing the same by using the anisotropic etch characteristics of III-V semiconductors where one orthogonal etch direction provides a natural channel for fiber positioning, the other orthogonal etch direction provides a reflecting surface for the redirection of optical energy between a fiber or waveguide and optical devices, and a non-selective etch provides a lens to focus optical energy on an optical device.
2. Description of the Prior Art
Compact and simple optical coupling systems for micro-optical devices are essential in optical communication systems. In addition, simplified assembly processes in packaging micro-optical coupling systems are very important in manufacturing low cost and reliable systems. An increasingly popular method for the coupling of optical energy between optical devices and systems is through the use of fiber and micro-optical lenses. Fiber provides an efficient transfer medium between optical devices by providing improvements in coupling efficiency and communication lag. Micro-optical lenses provide additional coupling efficiency by focusing divergent optical energy output from an optical fiber end. Present optical coupling systems use a variety of coupling schemes to obtain efficient coupling between micro-optical devices.
The publication "Packaging Technology for a 10-Gb/a Photoreceiver Module", by Oikawa et al., Journal of Lightwave Technology Vol. 12 No. 2 pp.343-352, February 1994 discloses an optical coupling system containing a slant-ended fiber 10 secured in a fiber ferrule 12 where the fiber ferrule 12 is welded to a side wall 14 of a flat package 16 and a microlens 18 is monolithically fabricated on a photodiode 20 where the photodiode 20 is flip-chip bonded to the flat package 16, as illustrated in FIG. 1. An optical signal 22 enters horizontally and is reflected vertically at the fiber's 10 slant-edge. The microlens 18 then focuses the optical signal 22 on the photodiode's 20 photosensitive area.
In the Oikawa publication, maintaining alignment between the fiber and the photodiode chip is essential for optimal coupling of the optical signal. Misalignment can occur as a result of mechanical stress to the fiber ferrule or thermal fluctuations of the entire system. In an attempt to overcome these factors, complex assembly and fabrication techniques are used. The fiber attachment is a complex ferrule attachment which seeks to optimize the mechanical strength of the attachment and therefore minimize the effects of fiber displacement. Because the photodiode chip is flip-chip bonded on the flat package a complex bonding machine is required for high-precision alignment. Finally, in order to provide a high optical coupling efficiency wide misalignment tolerances must be built in to the photodiode chip during fabrication to compensate for both displacement by the fiber attachment and deformation by temperature fluctuation.
Disclosed in U.S. Pat. No. 5,346,583 is a monolithic coupling system for optical energy transfer between a microlens and a fiber, as illustrated in FIG. 2. The configuration disclosed in patent '583 contains at least one preshaped photoresist (PR) microlens 24 formed on a surface 33 of a substrate 34 by standard photolithography steps and on an opposing surface 31 of the substrate 34 an optical fiber guide 26 is formed through standard photolithography steps. The fiber guide 26 is used to mount an optical fiber 28 such that the central axis 30 of the optical fiber 28 is substantially coincident with the central axis 32 of the PR microlens 24. While the proximity of the fiber 28 to the microlens 24 allows for efficient coupling of optical energy between the fiber 28 and an optical device, there are some significant disadvantages. First, the system is not very compact because of the orientation of the fiber 28 to the surface 31 of the substrate 34. More importantly, the PR microlens 24 cannot withstand variable temperature cycles and long-term reliability of the system would be an issue.
In many cases external lenses are used to couple optical energy between optical fibers or waveguides and optical devices. Examples of such coupling techniques are disclosed in U.S. Pat. Nos. 5,247,597; 4,653,847; 4,433,898; 4,875,750; and 5,343,546. Using external microlenses makes coupling extremely complex and in most cases unreliable.
As discussed, present optical coupling systems use a variety of coupling schemes to obtain efficient coupling between micro-optical devices. However, these schemes use many components, require a complicated assembly process, and are not compact. In addition, these components are typically made of different materials and have different thermal expansion coefficients. These differences can cause optical misalignment during temperature changes, which are common in military and space applications. Furthermore, when using discrete bulk optical components, the complexity of the assembly process is increased because there are more individual components to align. The greater the complexity the more assembly costs are increased and reliability decreased.
Based on techniques known in the art for optoelectronic coupling schemes, a monolithic alignment device for coupling optical energy between a fiber or a waveguide and an optical device is highly desirable.